1. Field of the Invention
This invention relates generally to electromagnetic motors and their control circuitry for power tools and devices, and more particularly, to a continuously variable frequency swinging armature motor and drive for improving the performance of such power tools and devices.
2. Description of Related Art
In the design of power tools and other work devices and machines, it is a continuing design goal to improve performance parameters to enhance user operation and productivity. In such devices as airless paint sprayers, high pressure washers, electric caulk guns, speed staplers, low volume water or air pumps, portable high pressure air compressors, power chisels and scrapers, among others, such parameters may include increasing output volumes, increasing user adjustability of output, increasing operating frequency ranges, and reducing acoustic noise, heat, and weight.
The ability to improve these parameters, particularly the output volume (e.g., of a paint sprayer), without increasing the size and power of the motors and pumps within such devices would be highly advantageous.
In devices using electromagnetic-based swinging armature motors, it has been found that the electromagnets that use single windings can be designed to improve performance of the devices. Such devices that use a single winding are usually powered directly from an alternating current (“AC”) line. This allows the armature to close at twice the line frequency, i.e., one time for the positive one-half of the AC waveform and one time for the negative one-half of the AC waveform. For a 60 Hz line, for example, the armature will move, open, and close 120 times per second with such an arrangement. Control over output volume (or other work done) is commonly accomplished by limiting how far the armature can move. This is sometime done by an adjustable mechanical stop. In other words, in order to control the output volume, you have to limit how far the armature can move, which is commonly accomplished by providing an adjustable mechanical stop. It would be preferable to electronically adjust the output, not mechanically.
Using a sine wave voltage to power the electromagnet is generally inefficient because the waveform has to fall to near zero volts before the armature can start to move to its open position. Therefore, there is less time for the pump's piston to fully open, and thus, there is less time for paint to enter into the pump section or reservoir of a typical paint sprayer. In addition, the armature also experiences a so-called bounce when it hits its open position stop. The bounce increases the settling time of the armature, thereby decreasing the overall output volume of the paint delivered from the typical paint sprayer. Therefore, it has been found that using other waveforms can improve electromagnetic motor performance. Also, where a DC signal is used to power an electromagnetic-based swinging armature motor, there is a need to have a current signal with a pulse width and associated circuitry that will allow the magnetic field generated by the electromagnet to collapse quickly and store the resulting energy within the circuit.
Accordingly, there is a need in the industry for a variable frequency driven swinging armature motor that can be continuously driven over a wide range of frequencies. Previous frequency adjustment solutions provided a circuit to selectively skip cycles or half cycles of the AC waveform. See e.g., U.S. Pat. Nos. 4,517,620 and 4,705,995 to Hans-Joachim Boll, the disclosures of which are hereby incorporated by reference in their entirety. Typically, this was accomplished by using a triac-type device (e.g., two thyristors wired in series, but pointing in opposite directions) that turns on and off at the zero crossings. Because the minimum period that can be skipped is one-half of a 60 Hz cycle, the frequency can only be reduced in 30 Hz increments while maintaining steady symmetrical output pressure and flow in the associated pump, which translates into drive frequencies of 120 Hz, 90 Hz, 60 Hz, and 30 Hz. Such frequency control limitations are commonly recognized as shortcomings in the industry. Therefore, a continuously variable frequency driven swinging armature motor would provide numerous benefits to the industry. Such a motor would provide more versatility and increase control in many applications. And for any applications where the lowest desired frequency is higher than the normal AC line frequency (i.e., 50–60 Hz), the electromagnet can be made physically smaller and still generate the same force, or alternatively, generate more force with the same size of electromagnet.
In accordance with the foregoing, an electromagnet based motor and associated circuitry incorporating improvements not heretofore employed for use in power tools and devices would be highly desirable.